1. Field of the Invention
The present invention relates to an optical component, optical amplifier and method of controlling an optical amplifier characteristic and may be applied for example to an erbium-doped fiber amplifier (EDFA) and to an optical component used to construct the EDFA.
2. Description of Related Art
An EDFA is an optical device comprising a plurality of optical components. FIG. 1 is a block diagram showing the layout of a conventional EDFA in which the forward pumping method is adopted.
In FIG. 1, pumping light from a pumping source (for example LD) 6 is sent through a WDM (Wavelength Division Multiplexing) coupler 2 to an erbium-doped optical fiber (EDF) 3 and is absorbed thereby, creating a highly inverted distribution therein. When input light (input signal light) is directed on to EDF 3 through isolator 1 and WDM coupler 2 in a condition with pumping light directed on to EDF 3, this signal light is progressively amplified by the stimulated discharge effect, and this amplified signal light is emitted as output signal light output light (output signal light) from isolator 4. The optical amplification characteristic of the output light that is output from isolator 4 shows wavelength dependence and in order to compensate for this wavelength dependence the optical signal is output through a filter 5 consisting of for example a fiber grating (FBG).
The specification of the various optical components that are employed in an EDFA depends on whether the input optical signal that is input by the EDFA is a single-wavelength optical signal (S signal) or a Wavelength Division Multiplex signal (WDM signal).
Also, pumping source 6 is typically constituted by a semiconductor laser (LD). When the power or wavelength of the pumping light from this pumping source (LD) 6 changes, the amplification characteristic in the EDF changes. Conventionally, in order to keep the pumping wavelength and pumping optical power constant, the pumping source 6 is provided with a temperature compensation construction so as to maintain constant temperature of the source 6. However, temperature compensation is not effected in respect of the other optical components.
Further, if the desired quality characteristics of the EDFA were not obtained for example on inspection on shipping, due to product variability of the various optical components, in order to obtain the desired quality characteristics of the EDFA, the following methods were adopted. The various types of characteristics include for example the optical amplification characteristic, wavelength-dependent output power difference, noise factor, and filter characteristic etc.
(1) Alteration of the amplification factor could be achieved by replacing some or all of the isolator 1, WDM coupler 2, EDF 3, isolator 4 or filter 5 components. For example, if the isolator 1, WDM coupler 2, isolator 4 and filter 5 were made replaceable, they could be replaced by components of different optical power loss, and if EDF 3 were made replaceable, it could be replaced by a component of different amplification factor.
(2) If it is desired to alter the wavelength-dependent output power difference when the WDM signal is amplified, this can be done by changing EDF 3, which constitutes the amplification medium, or by changing filter 5 to one of a different optical band pass characteristic.
(3) Improvement of the noise factor (NF) can be achieved by replacing EDF 3 by one of shorter length. Or this could be achieved by replacing pumping source (LD) 6 by one of larger pumping power, replacing isolator 1 by one of low power loss, or replacing WDM coupler 2 by one of low power loss.
As described above, to alter the quality characteristic of a conventional EDFA, the most direct and effective method was to replace optical components. However, this gave rise to the following inconveniences, after the EDFA was assembled as an optical amplifier.
(1) Alteration of the amplification factor is not easy since it involves replacement of optical components wherein optical fibers are fixed usually by means of adhesive. This is also inconvenient since, in order to achieve high output, it is necessary to change to an LD (pumping source) of large pumping power and/or to change the EDF itself.
(2) Alteration of the output characteristic at different wavelengths (i.e. wavelength-dependent output power difference) in a WDM signal is inconvenient since it is necessary either to alter the filter (for example FGB) for this characteristic, or to change the EDF itself.
(3) Improvement of the NF is inconvenient, since this must be effected by changing to an EDF 3 of shorter dimensions or by replacing pumping source (LD) 6 by one of larger pumping power, replacing isolator 1 by one of low power loss, or replacing WDM coupler 2 by one of low power loss.
(4) Temperature control is not performed in respect of optical components other than pumping source (LD) 6. The characteristics of these optical components have characteristic temperature dependencies. It is therefore a problem that, when the ambient temperature changes, the quality characteristic of the optical components departs from the normal-temperature characteristic on which the design was based and that this may therefore cause the quality characteristic of the EDFA as a whole to depart from the set characteristic.